The present invention is directed to the manufacture of a fishing net made of acoustically visible material to cetaceans, thereby preventing the incidental capture of cetaceans in the netting.
The inadvertent capture and destruction of cetaceans has been a matter of concern for many years. The numbers captured are in the hundreds or even thousands annually worldwide. Cetaceans swim into nets, become entangled, and die because they cannot reach the surface to breathe. The problem is mostly associated with the use of gillnets, which are fished either on the bottom or the surface.
Fishing nets for commercial fisheries are conventionally made either from a single filament (monofilament) or several filaments (multifilament) woven together. The woven material is called a twine or web. Lines and weights and floats are then attached to the twine or web to meet the particular requirements of the net. The different styles of net are termed gillnet, trawl, seine, or weir, depending on the specific construction and use.
The term cetacean refers to mammals living in the world""s oceans and rivers and includes various types of dolphins, porpoises, and whales. One characteristic of cetaceans is that they are able to echolocate, meaning they generate sound which radiates outward, and upon striking an object, is reflected back. The mammals use this device for locating food and navigating.
There are several theories regarding why cetaceans are captured in nets. One theory is that the nets are largely invisible to the cetaceans and they swim into them, even though they are echolocating, before they recognize their presence. Yet another theory says that they can identify the presence of the nets but they do not recognize them as something posing a threat. Another theory to account for cetacean capture is the animals swim into the nets because they are not using their echolocation system at the time of their capture.
Several approaches have been taken to end the capture of cetaceans. A concept that has been undergoing evaluation at several sites around the world is to use sound as a way of keeping cetaceans out of nets. A series of pingers are placed at regular intervals along the nets. Pingers are mechanical devices that emit sound, alerting cetaceans. Studies over the years have shown them to be effective in reducing cetacean capture.
There are several problems with the pingers, however. They are relatively costly and would increase significantly the cost of fishing. Furthermore, the pingers are battery powered and the batteries must be changed periodically, which is not a trivial matter since the pingers must be built so that they will not leak. Also, pinger failure leaves portions of the net unprotected. Finally, there is the problem that the pingers are intrusive in the environment because they are effective by making noise in the environment. There are concerns regarding whether migratory patterns of cetaceans might be altered as a result of the use of pingers.
In U.S. Pat. No. 5,349,774, the sound of feeding killer whales is played on a transducer under water in the vicinity of a mixed school of dolphin and tuna to selectively repulse the dolphin. In U.S. Pat. Nos. 5,251,187 and 5,117,572, an apparatus for steering the dolphins away from tuna is described, which also uses the sound of feeding killer whales.
There is a very sparse record of attempts to identify a passive system that would avoid the capture of cetaceans. A passive system is one in which there is no sound, light, smell or taste that impinges into the environment to provide a means of preventing cetacean capture.
Japanese application (JP 52009067) discloses making a pressure-resistant foamed material containing epoxy resin and used in floats for fishing nets. The intent was to alter the acoustic reflectivity of the floats to facilitate their detection by the mothership, not to solve the problem of capture of cetaceans. Studies of cetaceans in captivity have shown that they can detect large metal objects with their echolocation system.
The academic community is involved with the problem of cetacean capture, and the development of passive systems. One of the approaches to come from this sector is to weave a hollow core monofilament periodically into the net. Gillnets equipped with hollow core monofilament were tested alongside standard monofilament in a salmon mothership fishery, and it was found that there was no measurable drop in the rate of cetacean capture (W. W. Au and L. Jones, Mar. Mamm. Sci., 1991, vol 7, no 3, pp. 258-273; D. Hembree and M. B. Harwood report (Rep. Int. Whal. Commn. 37, 1987, pp. 369-373).
Hembree and Harwood disclose fishing for 126 sets using a 500 m net with 150 m of bead chain looped at 8 m. A similar net was modified with 56 m of 6 mm airfilled plastic tubing. Trials using commercial gillnet vessels established that neither the bead chain nor the plastic tubing had a significant impact on the dolphin by-catch.
Because of the failure of all of these approaches, one published position is that the only solution is to close certain areas of water to gillnetting (S. M. Dawson, Mar. Mamm., Sci., 7(3), pp. 274282, 1991). There have been several closures to gillnetting of waters within the Gulf of Maine every year. These closures cause great financial hardship to commercial fishermen, because it restricts severely the number of days they can fish over the course of a year.
The fundamental assumption being made in all of these potential solutions is that the net itself is not changed. Standard netting is modified by attaching materials to it. These materials include pingers or pieces of chain or hollow-core plastics, but the material used to make the net itself is not addressed. What has been lacking is an approach that would allow the net itself to be changed into a more reflective material, especially to frequencies detected by members of the cetacean family.
The inventors have discovered materials which, when incorporated into a monofilament, render the monofilament more acoustically reflective at frequencies used by cetaceans, at least in the 40-230 kHz range. By making the entire net more reflective, the net becomes more visible to cetaceans.
There are no prior examples of fabricated thermoplastic resin compositions that demonstrate acoustic reflectivities above the intrinsic reflectivity of the polymer of the thermoplastic material. Indeed, the concept of making and/or using a thermoplastic material with enhanced acoustic reflectivity is without precedent.
The acoustic reflectivity of an object can be measured by using a transmitter to send a signal directed at the target, then measuring the intensity of the signal returned. For a complete explanation of the experimental aspects see a report entitled, xe2x80x9cMonofilament Gill Net Acoustic Studyxe2x80x9d, by Elbert A. Pence, National Mammal Laboratory, Contract 40-ABNF-5-1988, which is incorporated herein by reference in its entirety. The target size of any object, according to Pence, is comprised of two factors, the geometrical target size and the impedance mismatch. Mr. Pence also reports on a series of tests on nylon monofilament and metal wire, determining that the metal wire was significantly more reflective of the signals. There is no example of prior art in which substances were added to a thermoplastic material to enhance acoustic reflectivity.
Adding substances to plastics is normal practice in the commercial art. These substances are added to reduce the cost of the finished product, to increase stiffness, or improve wear properties, for example. A general reference is J. Milewski, Handbook of Fillers and Reinforcements for Plastics, Van Nostrand Reinhold, New York, 1978, p. 66, which is incorporated herein by reference in its entirety.
The present invention is concerned with increasing the acoustic reflectivity of a thermoplastic resin, and more particularly with increasing its acoustic reflectivity by placing various acoustically reflective additives in a polymeric matrix of the thermoplastic resin, wherein the additives change the reflectivity of the products produced from the polymeric matrix by several decibels (dB). The instant invention is also concerned with providing polymeric compositions that can be used in forming acoustically reflective fishing nets, and carrying out fishing methods by using the acoustically reflective fishing nets of the present invention.